The present invention relates generally to maneuver detectors for use in weapons control systems, such as guided missile control systems, and the like, and more particularly, to a maneuver detector that employs a dynamic inertial coordinate system that may be used during medium pulse repetition frequency (MPRF), high pulse repetition frequency (HPRF), and interleaved track-while-scan (TWS) tracking modes of a target tracker.
Conventional maneuver detection methods have heretofore been used to derive a maneuver indication. A maneuver detector has been developed that is used for adjusting range-rate gates which compares a current range-rate to saved values from three previous frames, and indicates a maneuver if the difference for a comparison is greater than a threshold value. In general, because its range/bearing coordinate system rotates, a range acceleration exists for a target that is not accelerating relative to an inertial coordinate system, i.e., one that moves in a straight line at a constant velocity. For aircraft and target velocities of 1000 fps, such a maneuver detector might be triggered by such a target at about 15 nautical miles. These coordinate system accelerations can be corrected for in a Kalman tracker by "line-of-sight-rate aiding". A similar correction must be made in the maneuver detector in order to keep coordinate system-induced accelerations from appearing as (or masking) maneuvers. This method relies on the measured value of range-rate, and does not directly allow for the position-derived maneuver detection required for medium pulse repetition frequency (MPRF).
A single measurement of a parameter (such as range-rate) and a prediction of the value of that parameter under non-maneuvering conditions has been used to calculate a residual (the deviation of that parameter from an expected value if there is no maneuver). Then, the probability of obtaining a deviation at least that large under the null hypothesis (that there is no maneuver) is estimated. The residual (observed value less predicted value) of range, range-rate, etc., is normalized by values of track and measurement errors and compared to a distance threshold. Single frame indicators and smoothed versions thereof use only a single frame of time over which the prediction acts to derive an error. Analyses of an APG-71 tracker manufactured by the assignee of the present invention has shown that position measurement errors tend to be of similar magnitude to the change in these parameters that can be expected to develop over a frame time due to a target maneuver. Furthermore, in APG-71 track-while-scan mode, the filters include acceleration models, and to the degree that these models accurately represent a target acceleration, the acceleration term is included in the prediction to next frame and therefore does not show up in the residual. Consequently, a special prediction without the acceleration term must be made to use this method.
The rate-of-change of filtered velocity states has been used to calculate an acceleration for a target, and the magnitude of this acceleration vector is used to indicate maneuvers. This method is used in a maneuver detector used in an AWG-9 weapons control system manufactured by the assignee of the present invention. As in the first-described method, this method does not directly allow for a position-derived maneuver detection required for MPRF. Also, filtered north, east, down velocity states are slow (relative to the observation) in reacting to a maneuver, and mix the larger variances from coordinates oblique to the line-of-sight to the target (e.g., azimuth rate) into measurements along the range coordinate (e.g., range-rate), thus diluting their accuracy.
Several measurements of a parameter have been used to obtain a curve fit to a non-maneuvering model, and a goodness-of-fit parameter is used to estimate the probability that the measurements are consistent with that model. Several measurements of a parameter have been used to obtain a curve fit to a model allowing a maneuver, and the significance of the maneuver terms is used to indicate whether there is a maneuver. Measurements have been compared to a set of models consisting of several possible maneuvers as well as a no-maneuver model (e.g., a bank of Kalman filters representing different maneuver hypotheses), and the best fitting model is chosen.
Accordingly, it is an objective of the present invention to provide for an improved maneuver detector that is simpler, more accurate, and more generally applicable. It is a further objective of the present invention to provide for a maneuver detector that may be used during medium pulse repetition frequency, high pulse repetition frequency, and interleaved track-while-scan tracking modes of a target tracker.